10: Examination of the General Somatosensory System

INTRODUCTION

I. INTRODUCTION TO TESTING GENERAL SOMATIC SENSATIONS

A. Special and general senses

The special senses consist of sight, smell, taste, hearing, and equilibrium. The general senses tested in the Standard Neurological Examination (NE) consist of touch, pain, temperature, position, vibration, and stereognosis. Unique receptors and unique central pathways mediate each of the special senses. Some skin receptors and somatosensory pathways serve one general sensory modality, but other receptors in the skin are polymodal.

B. Negative and positive sensory phenomena after lesions of central and peripheral sensory pathways

1. Abnormal or noxious sensations like pain may arise in two ways: from stimulation of receptors or from intrinsic disease of the nerves or central pathways. Disease of afferent nerve fibers centrally or peripherally causes not only negative or deficit phenomena, with lack of sensation, as you would expect, but also, paradoxically, may cause positive phenomena, with excessive sensation, i.e., pain and tingling (Nashold and Ovelmen-Levitt, 1991).

2. Nomenclature for deficits in superficial sensation

   1. Esthesia = touch or feeling, so hypesthesia = partial loss of touch, and anesthesia = total loss. Anesthesia is also used to mean lack of pain.

   2. Therm = heat, so thermhypesthesia = partial loss of temperature sensation, and thermanesthesia = total loss.

   3. Algesia = pain, so hypalgesia = partial loss of pain sensation, and analgesia (or anesthesia) = total loss.

3. Nomenclature for pain and other noxious irritative phenomena after disease of peripheral or central sensory pathways

   1. Hyperesthesia, hyperalgesia, and hyperthermesthesia refer to an increased sensitivity to touch, pain, and temperature, respectively. For example, after a burn of an area of skin, even slight exposure to heat may cause intense pain, i.e., hyperthermesthesia.

   2. Paresthesias and dysesthesias are uncomfortable sensations of numbness, tingling, pins and needles, or burning pain short of neuralgia or causalgia. Paresthesias describes such sensations when they accompany a normal external stimulus to the skin. Dysesthesias describes their spontaneous occurrence without any obvious external stimulus (Wartenberg, 1953). (Unfortunately some writers reverse the definitions.) You will notice paresthesias and dysesthesias if you study your own sensations when recovering from a local anesthetic for a dental procedure or after having sat too hard on your own sciatic nerve, causing it to "go to sleep."

   3. Hyperpathia means an extreme overresponse to pain. Hyperpathia associated with a raised pain threshold is called anesthesia dolorosa.

   4. Neuralgia means multiple, very severe, electric shock-like pains that radiate into a specific root or nerve distribution. Examples include trigeminal neuralgia and post-herpetic neuralgia that commonly follow herpetic infection of a dorsal root ganglion (Gilden et al., 1991). The neuralgias may or may not alter the ...